Kin recognition refers to animals' capabilities to distinguish between close genetic kin and non-kin. In evolutionary biology and in psychology, such capabilities are presumed to have evolved to serve the adaptive functions of kin altruism (see kin selection) and inbreeding avoidance. Apart from a few exceptional cases (e.g., the green-beard effect), kin recognition is achieved via several cue-based mechanisms (e.g., imprinting, phenotype matching). Because kin recognition is cue based, errors sometimes occur. A well-known example is the Westermarck effect in which unrelated individuals who spend their childhood in the same household find each other sexually unattractive.